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Industrial Ethernet S7-300/400 CPs -- Product information -- System description (protocols)
Which technical data applies for the SIMATIC Modbus/TCP blocks and for which CPUs and CPs is it released.
How many devices can you connect to an S7-300/S7-400 using the Modbus/TCP protocol?
When compiling a CFC chart, why is the error message "Time stamp conflict" displayed in connection with the Modbus/TCP FBs for S7-400 CPUs?
How many licenses are required for the Modbus/TCP block in the S7-300/S7-400?
Which IO controllers and IO devices support the following functions in STEP 7 (TIA Portal): IRT, prioritized startup, MRP, MRPD, PROFIenergy, Shared device, MSI/MSO, I device, Isochronous mode, System redundancy and Option handling?
How are the communication resources assigned in the S7-300?
How do you load the 3D, CAx and ePLAN data for the SIMATIC products?
What are the differences between OPEN MODBUS / TCP Redundant V1 and OPEN MODBUS / TCP Redundant V2?
What should you watch out for when you enable the "SNMP" function in the configuration of the security module?
Which security modules support the dynamic DNS function (DDNS)?
What should you watch out for when enabling and using the security functions of CP343-1 Advanced and CP443-1 Advanced?
What should you watch out for when upgrading from previous modules to the successor module CP443-1 or CP443-1 Advanced?
How do you verify the authenticity of the Siemens modules CP343-1 and CP443-1?
What should you watch out for when operating a port of CP343-1, 6GK7343-1CX10-0XE0 and 6GK7343-1EX30-0XE0 with firmware V2.1 or higher, with 10 Mbit/s half duplex?
Which IO controllers and IO devices support the following functions in STEP 7 V5.5: IRT, prioritized startup, MRP, MRPD, PROFIenergy, Shared device, I device, sochronous mode, System redundancy and Option handling?
Where do you procure the firmware extension for the CP 343-1 ERPC for database connection?
Which SIMATIC S7-300/S7-400 modules support the NTP time-of-day message and how do you activate this kind of time synchronization?
Which modules can you use when implementing software redundancy?
Which types of connection/protocols do the S7-300/400 CPUs and the CPs support by default?
Which PROFINET nodes support automatic commissioning (device replacement without interchangeable medium) in the case of topological configuration and device replacement without topological configuration?
Which PROFINET nodes support the extended PN diagnostics and what do you have to configure?
Using C-PLUG with network components and SIMATIC CPs
What properties have the communication services and protocols that can be used in the Industrial Ethernet network?
Behavior of the communication processors during data transfer via TCP connections without RFC 1006
What should you watch out for with the ICMP Redirect (Internet control message protocol) function regarding the IP address of the router used for Industrial Ethernet CPs?
What should you pay attention to with the autonegotiation function for negotiating the LAN mode in Industrial Ethernet CPs?
What is the connection between subnet masks and IP addresses with regard to subnetting and supernetting (Classless Inter Domain Routing CIDR)?
Which modules support the S7 routing function?
In the case of open TCP/IP communication via Industrial Ethernet using T blocks with IE CPs and CPUs, why are different amounts of messages sent when the maximum length of 1452 bytes of user data is transferred?
What properties, advantages and special features does the ISO protocol offer?
What properties, advantages and special features does the UDP protocol offer?
What properties, advantages and special features does the ISO-on-TCP protocol offer?
What properties, advantages and special features does the S7 protocol offer?
What properties, advantages and special features does the TCP protocol offer?
What are the requirements for disabling the PROFINET CBA interface for CP443-1EX40?
"Time Wait" with TCP connection setup
What are the different data block lengths for the various protocols of Send/Receive communication?
What is RFC1006 and what do I need this service for?
Meaning and function of the "Keep Alive" in TCP connections
What properties, advantages and special features does the UDP protocol offer?
Part number:

The UDP protocol was introduced to transfer data quickly and simply. The UDP protocol is on Level 4 (Transport Layer) of the ISO-OSI reference model and is thus also based on the IP layer (Layer 3). The recipient of data is therefore addressed using IP addresses. Only minimal administration information is add to the data packet to be sent, so that the data throughput is higher compared with TCP/IP.

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Fig. 01 - Position in the ISO-OSI reference model

Properties of the UDP protocol:
Considering the requirement to transfer data rapidly, the UDP protocol only provides basic functions. Thus data can be exchanged between communicating partners with a minimum of effort. There are no safety mechanisms as with TCP/IP. The UDP protocol is connectionless and packet-oriented.

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Fig. 02 - Structure of a UDP datagram

Socket and ports:
Several processes can run simultaneously on multitasking operating systems, whereby each process can provide multiple services. They should be able to be addressed separately.
For this reason, special interfaces, or ports, are defined for data communication both in the TCP/IP and the UDP protocol. This expansion of the host-to-host transfer to a process-to-process transfer is termed application multiplexing and demultiplexing.

Together with the IP address for a station, the port numbers (local and remote) form a socket which is defined as the unique address of the user program in the entire network. Therefore, any of a process's services can be addressed to a station within a network using a socket.

In the SIMATIC environment, multiple "simultaneous" processes or user programs cannot be created on one CPU. However, multiple communication jobs may be initiated simultaneously. In order to facilitate unique addressing here as well, assignment must take place via the connection configuration. Connection types (in the SIMATIC configuration NetPro):
Connection type Description
Specified UDP connection
- local node and connection partners are firmly configured
- the connection partner can be inside or outside the STEP 7 project
Unspecified UDP connection 1) - only the local node is defined in the connection configuration
- addressing of the partner is via port and IP address upon block call
Broadcast 1) - an active node sends data to all other nodes
Multicast 1) - an active node sends data to one firmly configured group of nodes
 1) Cannot be configured with CPUs for open Ethernet communication

Performance data and quantity framework of the UDP protocol:
Criterion Performance data Additional information
Definition RFC 768 Non-proprietary
Transmission medium Cable, fiber-optics, radio  
Transmission rate Independent of network, up to 1 Gbit  
Connectable devices and access procedures - Point-to-point
- Broadcast
- Multicast
Permissible processes:
Acknowledgment The protocol only acknowledges successful sending of the data into the network and not the arrival of the data at the target station. The user program must take care of securing consistency and data preparation.  
Data volume 1 - 2048 bytes  
Number of possible connections  Up to 16 per S7-300 CP
Up to 64 per S7-400 CP

Advantages of the UDP protocol:

  • very rapid data transfer
  • very flexible, can be well implemented with third-party systems
  • routing-compatible
  • multicast-compatible / broadcast-compatible
  • suitable for small-to medium volumes of data (<= 2048 bytes)

Disadvantages of the UDP protocol:

  • lost data packets are not sent again
  • data packets with checksum errors are discarded and not requested again
  • multiple settings of individual packets is possible
  • the order of arrival of the packets at the receiver's end cannot be predicted
  • data is transferred packet-oriented (not stream-oriented)
  • broadcast function can only be used in send direction


  • Entry ID 20983558 provides a complete example of a UDP multicast communication.
  • General information on communication via SIMATIC S7 is available in Entry ID 20982954.

 Entry ID:26484229   Date:2007-09-24 
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